This invention relates to a patterned multi-layer structure formed by reactive sputter etching, especially to an optical filter comprised of a multi-layer filter structure and more particularly to a color separating stripe filter for a pickup device.
The basic color television camera system contains three photoelectric pickup devices which respectively pick up red, green and blue optical component signals from an object. To miniaturize the camera, a single tube or 2-tube color camera system is used. As the photosensitive pickup devices, a solid state device such as a CCD (charge coupled device) is utilized in lieu of a tube. When the camera system contains only one or two pickup devices as mentioned above, a stripe filter has to be located in front of the device. The stripe filter acts to separate red, green and blue components of the light from an object and to send these components to the pickup device to produce electric signals corresponding to the respective color components. There are various structures possible for the stripe filter. A typical example of a stripe filter is shown in FIG. 1. This filter is applied to a single tube color camera and has chromatic filter layers 11 and 12 on a transparent or semitransparent substrate such as a glass plate. The chromatic filter layer 11 which passes blue and green and reflects red is called a cyan filter. The chromatic filter layer 12 which passes green and red and reflects blue, is called a yellow filter. Therefore the crossing portions 13 at which the layer 11 and the layer 12 overlap each other can transmit green only. Symbols R, G and B denote respectively red, green and blue components which cam pass the respective portions. These chromatic filter layers need chemical and thermal stability to be applied to a pickup device such as a vidicon. So a material of high melting point such at TiO.sub.2, SiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.3 or CeO.sub.2 is usually used for the components of these filters. High refractive index layers and low refractive index layers are alternately superimposed to and form a multi-layer optical filter structure in order to pass the desired chromatic light. Conventionally a reverse etching method is used to make such a stripe filter because it is difficult to etch directly a multi-layer structure composed of said high melting point materials after depositing it on a substrate. The reverse etching method is a method comprising the steps of forming a desired negative pattern of metallic film on a substrage, depositing a multi-layer filter structure on the metallic pattern, etching the metallic film with an etchant and removing mechanically the multi-layer structure applied to the metallic film. FIGS. 2a and 2b are a photograph and an enlarged perspective view of the stripe filter thus obtained. As seen from FIG. 2b, the upper edges 23 of the etched surfaces of the multi-layers 21 and 22 are not straight but zigzag shaped. The width of the stripes of the filter is about 17 microns for a 2/3 inch pickup tube. Tolerable amplitude (a) of zigzag to the stripe width W is represented by EQU a=(1/10)w (1)
When a is larger than 1/10 of W, the noise becomes large and consequently it is difficult to get a clear picture. It is very hard to satisfy equation (1) by using reverse etching. Moreover the wet etching method used in reverse etching has some drawbacks contingent to metallic film evaporation and etching processes. Namely these drawbacks are as follows.
(1) Etching residue 31 (FIG. 3) of the metallic film 32 for the cyan filter or paar reverse etching which leaves the multi-layer to be removed occurs. FIGS. 3a and 3b are a photograph and its sketch of the etching residue of the metallic film. Reference numeral 33 denotes a striped yellow filter.
(2) Blots are often generated along the edges of the stripes owing to the reaction between the metalic film and the multi-layer filter structure because the substrate must be heated in the evaporation process for forming the multi-layer structure to stabilize the layer.
(3) Splashing occurs when the metallic film is evaporated. This metallic splashing causes scratches on the filter.
(4) Insufficient adhession of the metallic film is easily caused by inadequate substrate temperature during the evaporation process which causes exfoliation of the film.
(5) The edges of the stripes are not straight because the shape of the edge line is determined by the grain size of the metallic stripes.